filepp - an adaptation
and extension of the C preprocessor for general-purpose use. Written in Perl.
Open source (GPL-2-or-later).

chpp (Chakotay
Preprocessor) - a powerful preprocessor that aims to be non-intrusive,
and which can be considered a full-fledged programming system. Has been
unmaintained since 1999. Open source (GPLv2).

General-purpose Template Systems

Template Toolkit - a flexible
and highly extensible template processing system for Perl. Open source
(same terms as Perl).

ClearSilver - a language-agnostic
and fast templating system written in C.

Parser Generators

ANTLR - “ANTLR, ANother Tool for Language
Recognition, is a language tool that provides a framework for constructing
recognizers, interpreters, compilers, and translators from grammatical
descriptions containing actions in a variety of target languages.” Open Source
(3-clause BSD licence).

Parse-RecDescent
- a parser-generator for Perl 5. Open source (same terms as Perl).

Marpa - a parser than aims
to be able to parse everything in BNF. Open source (LPGL-version-3-or-later).

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